Can you send me a link to those images that failed you, ill try and fix it. It has something to do with exif data and color representation. I thought i fixed that issue but appears not. If you use paint and save as a bmp, then open the bmp and save as jpg it will fix it, or you can change the size so its not 256x256 and itll force conversion.Yeah, couldn't figure out why I so just went back to an earlier nro2nsp.
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It was after creating the NSP and installing on the Switch that i'd see only a ? icon even though I added an image.